Nitrogen producing cryogenic air separation unit with excess air circuit

ABSTRACT

A nitrogen producing cryogenic air separation unit with an excess air circuit is provided. The nitrogen producing cryogenic air separation unit is capable of producing high pressure gaseous nitrogen without the use of a nitrogen product compressors and is also capable of producing high rates of liquid nitrogen without adding additional compression stages in the main air compressor and/or without a nitrogen recycle compressor.

TECHNICAL FIELD

The present inventions relates to a nitrogen producing cryogenic airseparation unit, and more particularly, to nitrogen producing airseparation units with an excess air circuit capable of producing highpressure gaseous nitrogen without the use of a nitrogen productcompressor and that is also capable of producing high rates of liquidnitrogen without adding additional compression stages in the main aircompressor and/or without a nitrogen recycle compressor.

BACKGROUND

Prior art nitrogen producing air separation units attempting to producehigh pressure gaseous nitrogen product without use of a nitrogen productcompressor and also capable of producing high levels or rates of liquidnitrogen typically required raising the nitrogen column pressure byincreasing the pressure of the incoming feed air, typically by addingadditional stages of feed air compression to the main air compressorarrangement.

Because the nitrogen column in such nitrogen producing air separationunits is operating at higher pressures, the waste nitrogen from thecolumn is also at a higher pressure and capable of generating morerefrigeration when the waste nitrogen gas is expanded in the wasteexpander. The additional refrigeration allows the air separation unit tobe capable of greater rates of liquid nitrogen production. The maindisadvantage of this prior art solution is it requires more power fromthe main air compressor arrangement and entails a more difficultseparation process within the higher pressure nitrogen column requiringmore separation stages in order to reduce the power/recovery penaltythat arises from the higher column pressure.

To further increase the liquid nitrogen production rates in suchconventional nitrogen producing air separation plants, a motor-drivennitrogen recycle compressor and additional turbine/expander may also beadded to create a supplemental refrigeration circuit or supplementalsource of refrigeration. In some cases, the recycle compressor functioncan be included on the same machine in combined service with the mainair compressor. For such cases, a customized compressor design isrequired, which is an appreciable capital cost addition. In addition,the nitrogen producing air separation plants that employ conventionalsupplemental refrigeration circuits incur additional capital costs thatresult from larger heat exchangers, distillation columns, and pipingcomponents due to the high flow recirculation.

What is needed therefore is an improved nitrogen producing airseparation unit and cycle that is capable of producing high pressuregaseous nitrogen without use of a nitrogen product compressor andcapable of producing high liquid nitrogen production rates (e.g. morethan 30% of the nitrogen product as liquid nitrogen) while eliminatingthe need for adding additional compression stages to the combinedservice compressor or a separate recycle compressor and/or reducing thesize and number of additional compression stages to reduce theassociated capital costs.

SUMMARY OF THE INVENTION

The present invention may be characterized as a method of providingsupplemental refrigeration in a nitrogen producing air separation unitcomprising the steps of: (a) compressing and purifying the incoming feedair stream to produce a compressed, purified air stream; (b) splittingthe compressed, purified air stream into an excess air stream and acompressed, purified feed air stream; (c) compressing the excess airstream in one or more excess air compressors to a pressure greater thanabout 24 bar(a); (d) cooling the compressed, purified feed air stream ina heat exchanger to produce a fully cooled feed air stream that isdirected to a distillation column system and also cooling a firstportion of the further compressed excess air stream in the heatexchanger to produce a liquid air stream that is directed to thedistillation column system; (e) expanding a second portion of thefurther compressed excess air stream in an excess air expander toproduce an excess air exhaust stream; (f) combining the excess airexhaust stream with a waste stream from the distillation column systemto produce a combined excess air and waste stream; (g) expanding thecombined excess air and waste stream in an excess air and waste expanderto produce a waste exhaust stream; and (h) warming the waste exhauststream in the heat exchanger to provide supplemental refrigeration tocool the first portion of the further compressed excess air stream andcool the fully cooled feed air stream.

The nitrogen producing air separation unit is configured to receive anincoming feed air stream and produce a high pressure gaseous nitrogenproduct, preferably without a nitrogen product compressor and alsoproduce a liquid nitrogen product. To produce such nitrogen products,the distillation column system of the air separation unit comprises atleast one nitrogen column and at least one nitrogen condenser.Preferably, the distillation column system preferably includes onenitrogen column and one nitrogen condenser. Alternative embodimentsinclude arrangements where the distillation column system comprises adual nitrogen column arrangement with one or two nitrogen condensers.The waste stream from the distillation column system is preferably awarmed, boil-off vapor stream from one of the nitrogen condensers.

The present invention may also be characterized as a nitrogen producingair separation unit configured to receive an incoming feed air streamand produce a high pressure gaseous nitrogen product and a liquidnitrogen product, and further configured to be capable of taking morethan 30% of the nitrogen product as liquid nitrogen product. Thenitrogen producing air separation unit comprises: (i) an excess aircircuit that includes an excess air stream diverted from the compressed,purified feed air stream, preferably at a location downstream of themain air compression and purification system; (ii) one or more excessair compressors configured to further compress the excess air stream toa pressure greater than about 24 bar(a), wherein a first portion of thefurther compressed excess air stream is directed to a distillationcolumn system having at least one nitrogen column; (iii) an excess airexpander configured to expand a second portion of the further compressedexcess air stream to produce an excess air exhaust stream; and (iv) awaste and excess air expander configured to receive a vapor stream fromthe nitrogen condenser and the excess air exhaust stream and expand thevapor stream from the nitrogen condenser and the excess air exhauststream to produce a waste exhaust stream. The waste exhaust stream isconfigured to be warmed in the heat exchanger against the compressed,purified feed air stream and the excess air stream to provide thesupplemental refrigeration necessary to support the high liquid productmake.

In one embodiment, the one or more excess air compressors furthercomprise a motor driven booster compressor configured to receive andfurther compress the excess air stream and one or more boostercompressors driven by the waste and excess air expander and excess airexpander. In this embodiment, the one or more excess air compressorsconfigured to further compress the excess air stream to a pressurebetween about 34 bar(a) and 55 bar(a).

In another embodiment, the one or more excess air compressors furthercomprise a first booster compressor operatively coupled to and driven bythe waste and excess air expander and a second booster compressorarranged in series with the first booster compressor, the second boostercompressor operatively coupled to and driven by the excess air expander.The two booster compressors arranged in series are configured to furthercompress the excess air stream to a pressure between about 24 bar(a) and35 bar(a).

In yet another embodiment, the nitrogen producing air separation unitcomprises a feed air booster compressor downstream of the main aircompression and purification system and configured to further compressthe incoming feed air, the feed air booster compressor operativelycoupled to and driven by the waste and excess air expander. In thisembodiment, the one or more excess air compressors further comprise twobooster compressors arranged in series and configured to receive andfurther compress the excess air stream wherein one of the boostercompressors is operatively coupled to and driven by the excess airexpander.

BRIEF DESCRIPTION OF THE DRAWINGS

While the present invention concludes with claims distinctly pointingout the subject matter that Applicant regards as the invention, it maybe better understood when taken in connection with the accompanyingdrawings in which:

FIG. 1 is a schematic process flow diagram of a conventional singlecolumn nitrogen producing cryogenic air separation unit that employs anitrogen recycle loop to provide supplemental refrigeration to allowhigher liquid nitrogen production;

FIG. 2 is a schematic process flow diagram of an embodiment of anitrogen producing cryogenic air separation unit in accordance with anembodiment of the present invention;

FIG. 3 is a schematic process flow diagram of another embodiment of thepresent nitrogen producing cryogenic air separation unit;

FIG. 4 is a schematic process flow diagram of yet another embodiment ofthe present nitrogen producing cryogenic air separation unit;

FIG. 5 is a schematic process flow diagram of still another embodimentof the present nitrogen producing cryogenic air separation unit;

FIG. 6 is a schematic process flow diagram of an embodiment of thepresent nitrogen producing cryogenic air separation unit where theexcess air and waste expander is generator loaded;

FIG. 7 is a schematic process flow diagram of another embodiment of thepresent nitrogen producing cryogenic air separation unit where theexcess air and waste expander is also generator loaded; and

FIG. 8 is a schematic process flow diagram of still another embodimentof the present nitrogen producing cryogenic air separation unit with adual column arrangement and where the excess air and waste expander isalso generator loaded.

DETAILED DESCRIPTION

As discussed in more detail below, the disclosed cryogenic airseparation systems and methods provide certain cost and performancebenefits over conventional nitrogen producing cryogenic air separationunits depicted in FIG. 1 . The various embodiments of the presentnitrogen producing air separation unit all utilize a uniquely configuredexcess air circuit. This excess air circuit includes multiple expandersand one or more booster compressors that allows the nitrogen producingair separation unit to produce high pressure gaseous nitrogen withoutthe use of a nitrogen product compressor. This disclosed excess aircircuits also allows the nitrogen producing air separation unit toachieve very high rates of liquid nitrogen production without addingadditional compression stages in the main air compressor and/or withouta separate nitrogen recycle compressor.

Turning now to FIG. 2 , there is shown a schematic illustration of thepresent nitrogen producing cryogenic air separation unit 10. In a broadsense, the depicted air separation unit includes a main feed aircompression train or system, an excess air circuit, a main heat exchangesystem, a distillation column system, and a nitrogen liquefactionsystem.

In the main feed compression train shown in FIG. 2 , the incoming feedair 22 is typically drawn through an air suction filter house and iscompressed in a multi-stage, intercooled main air compressor arrangement24 to a pressure that can be between about 6.5 bar(a) and about 11bar(a). This main air compressor arrangement 24 may include integrallygeared compressor stages or a direct drive compressor stages, arrangedin series or in parallel. The compressed air stream 26 exiting the mainair compressor arrangement 24 is cooled in aftercooler and then fed to apre-purification unit 28 to remove impurities including high boilingcontaminants. The pre-purification unit 28, as is well known in the art,typically contains two beds of alumina and/or molecular sieve operatingpreferably in accordance with a temperature swing adsorption cycle inwhich moisture and other impurities, such as carbon dioxide, water vaporand hydrocarbons, are adsorbed. One or more additional layers ofcatalysts and adsorbents may be included in the pre-purification unit 28to remove other impurities such as carbon monoxide, carbon dioxide andhydrogen to produce the compressed, purified air stream 29. Particulatesmay be removed from the feed air in a dust filter disposed upstream ordownstream of the pre-purification unit 28.

As shown in FIG. 2 , the compressed, purified air stream 29 may be splitinto a plurality of air streams, including an excess air stream 31 and acompressed, purified feed air stream 33. Excess air stream 31 may befurther compressed in one or more excess air compressors, including amotor driven compressor 37A and a pair of booster compressors 37B, 37Cand subsequently cooled in aftercoolers 39A, 39B, 39C to form a boostedpressure excess air stream 36. The boosted pressure excess air stream 36is then directed to the main heat exchange system which includes heatexchanger 52. A first portion 38 of the boosted pressure excess airstream is partially cooled and exits the heat exchanger at anintermediate location as a partially cooled excess air stream 38 exitsheat exchanger 52. The partially cooled excess air stream 38 is thenexpanded in expander 35 to produce exhaust stream 44 that is thenfurther cooled in the heat exchanger 52 to form the cooled excess airexhaust stream 45.

As discussed in more detail below, the cooled excess air exhaust stream45 is combined with a warmed waste stream 46 to form a combined excessair and waste stream 48. The combined excess air and waste stream 48 isthen directed to a waste and excess air expander 40 where it is expandedto form a waste exhaust stream 49 that is warmed in heat exchanger 52.In this manner, a portion of the refrigeration for the air separationunit 10 is thus provided by the expansion of the combined excess air andwaste stream 48 in expander 40 thus allowing a higher liquid nitrogenproduction by the air separation unit 10. The warmed waste stream 41exits the warm end of the heat exchanger 52 and may be used as a purgegas stream during regeneration of the adsorbents and other layers in thepre-purification unit 28.

In the embodiment of FIG. 2 , a motor driven compressor 37A is shownraising the pressure of the excess air stream 31 prior to its feed tothe first booster compressor 37B. The motor driven compressor 37Apreferably is only a single stage and its capital cost will be less thanthat of a separate recycle compressors shown in the conventionalarrangement depicted in FIG. 1 . The preferred turbine booster loadingconfiguration is as shown, with the first booster compressor 37B loadedby the waste and excess air expander 40 and the second boostercompressor 37C loaded by the excess air expander 35. The first boostercompressor 37B may require gearing between the expander 40 and boostercompressor 37B for an effective design. The speed of the second boostercompressor 37C matches quite well with the speed of the excess airexpander 35, so the machines may be directly coupled with no gearing andprovide an effective design.

Turning now to the compressed, purified and cooled air streams to bedirected to the distillation column system, a second portion of theboosted pressure excess air stream 32 is further cooled in heatexchanger 52 and exits the cold end of the heat exchanger 52 as a fullycooled liquid air stream 55. The fully cooled liquid air stream 55 isthen introduced into distillation column system, preferably at alocation proximate the bottom of the distillation column 65 in a singlecolumn arrangement as shown in FIG. 2 or in the bottom section of thehigher pressure column in a dual column distillation arrangement (See.FIG. 8 ).

The compressed, purified feed air stream 33 is also fully cooled in heatexchanger 52 and exits the cold end of heat exchanger 52 as a fullycooled feed air stream 56 that is also introduced into distillationcolumn system, preferably at a location several stages above the bottomof the distillation column 65 in a single column arrangement or severalstages above the bottom of the higher pressure column in a dual columnarrangement (See. FIG. 8 ).

Cooling of the second portion 32 of the boosted pressure excess airstream and the compressed, purified feed air stream 33 in the heatexchanger 52 to produce cooled air streams suitable for rectification inthe distillation column system is preferably accomplished by way ofindirect heat exchange with the warming streams which may include: awaste stream 59A from the distillation column system; a nitrogen productstream 57A from the distillation column system; the waste exhaust stream49 from the excess air circuit; and a recycle stream 58A from thenitrogen liquefaction system. The heat exchanger 52 is preferably abrazed aluminum plate-fin type heat exchanger. Such brazed aluminum heatexchangers are advantageous due to their compact design, high heattransfer rates and their ability to process multiple streams. They aremanufactured as fully brazed and welded pressure vessels.

The illustrated distillation column system includes a singledistillation column 65 and a main condenser-reboiler 75. Thedistillation column 65 typically operates in the range from betweenabout 7.5 bar(a) to about 17 bar(a). Fully cooled feed air stream 56 andliquid air stream 55 are fed into the distillation column 65 forrectification resulting from mass transfer between an ascending vaporphase and a descending liquid phase that is initiated by a nitrogenbased reflux stream. A plurality of mass transfer contacting elements,that can be trays or structured packing or other known elements in theart of cryogenic air separation are disposed within the distillationcolumn 65. This separation process within the distillation column 65produces a nitrogen-rich column overhead 66 and crude oxygen-enrichedbottoms liquid also known as kettle liquid which is taken as kettlestream 67. The kettle stream is preferably subcooled in subcooler 53 viaindirect heat exchange against: a first part of the nitrogen-rich columnoverhead 66 taken as the gaseous nitrogen product stream 57B; theboil-off stream or waste stream 59B from the main condenser-reboiler 75;and the recycle stream 58B from the nitrogen liquefaction system. Thesubcooled kettle stream 68 is directed to the main condenser-reboiler 75to condense a clean shelf nitrogen stream 69 taken as second part of thenitrogen-rich column overhead 66.

The condensation produces a liquid nitrogen stream 71 exiting the maincondenser-reboiler 75 that is separated into a first portion, referredto as the reflux stream 73, that is released into the distillationcolumn 65 to initiate the formation of descending liquid phase thereinand a second portion, referred to as the liquefaction feed stream 72,that is fed to the nitrogen liquefaction system.

The boil-off stream from the main condenser-reboiler 75 is a wastestream that is warmed in subcooler 53 and main heat exchanger 52. Thewarmed waste stream 46 is combined with the cooled excess air exhauststream 45 to form the combined excess air and waste stream 48 anddirected to the waste and excess air expander 40 where it is expandedwith the resulting exhaust stream directed to main heat exchanger toprovide the supplemental refrigeration necessary to allow higher liquidnitrogen production. Combining the warmed waste stream 46 with thecooled excess air exhaust stream 45 may occur outside the main heatexchanger, as illustrated or may occur within the brazed aluminum heatexchanger with the use of a large side header configured to receive bothstreams from their respective heat exchange passages.

In the illustrated embodiment, the nitrogen liquefaction system isdepicted as a subcooler 80 that is configured to subcool theliquefaction feed stream 73 to produce a subcooled liquid nitrogenstream 82. A liquid nitrogen product stream 84 is taken as a firstportion of the subcooled liquid nitrogen stream while the remainingportion 86 of the subcooled liquid nitrogen stream is used as thecooling medium in subcooler 80 after being let down in pressure. Thewarmed nitrogen stream exiting subcooler 80 is recycled to the main heatexchanger 52 as recycle stream 58A. The recycle stream exiting the mainheat exchanger 52 is a nitrogen vapor stream that may be recycled to themain air compression train or system.

The present system and method differs from the conventional high liquidmake nitrogen producing air separation units (See FIG. 1 ) in that adiverted excess air stream 31 is used to generate supplementalrefrigeration in order to eliminate the recycle compressors of theconventional high liquid make nitrogen producing air separation units(See FIG. 1 ). In order to most effectively utilize the excess airstream 31 it is necessary that the excess air stream be expanded over ahigh pressure ratio. This is accomplished by first raising the pressureof the excess air stream using one or more upstream motor-driven and/orbooster compressors and also by using at least two turbines/expanders.The boosting is preferably done with some or all of the boostercompressors absorbing the refrigeration energy of each turbine/expander.Also, the refrigeration needs of the present system allow for arelatively warm inlet temperature to the waste and excess air expander40 without creating an excessively pinched temperature profile in themain heat exchanger 52. Comparatively, the power consumption for thearrangement depicted in FIG. 2 is similar to the power consumption ofthe conventional high liquid production air separation unit shown inFIG. 1 at comparable feed air and nitrogen product flows.

Using a relatively warm inlet temperature allows the present system totake advantage of the higher energy a warmer excess air expanderprovides so that the pressure rise in the booster is higher. In order tofurther expand the excess air after the excess air expander 35 the flowis further cooled in the main heat exchanger 52 and combined with thewarming waste stream for expansion in the waste and excess air expander.Combining these streams and feeding the combined stream to the waste andexcess air expander 40 is an important and key feature of the presentsystem and method. By using the combined stream and the waste and excessair expander 40 for the second stage of expansion, there is no need fora third expander.

Turning to FIG. 3 , there is shown a schematic diagram of an alternateembodiment of the present system and method. Many of the features,components and streams associated with the nitrogen producing airseparation unit 11 shown in FIG. 3 are similar or identical to thosedescribed above with reference to the embodiment of FIG. 2 and for sakeof brevity will not be repeated here. The key differences between thenitrogen producing air separation unit 11 illustrated in FIG. 3 comparedto the air separation unit 10 in the arrangement shown in FIG. 2 is theabsence of the upstream, motor-driven booster compressor 37A andaftercooler 39A in the excess air circuit.

In the embodiment of FIG. 3 , since the small motor driven compressor37A is eliminated. the excess air flow must be increased to produce thesame liquid product rate. In this embodiment, there are capital costsavings realized by eliminating the small motor driven compressor 37Aand aftercooler 39A which is partially offset by a slightly higheroverall power cost required to compress the higher excess air flow. Notethat for a lower liquid make production rate, the power savingsincentive for the motor driven compressor of FIG. 2 is reduced as thepower penalty associated with the embodiment of FIG. 3 becomes smalleras the liquid production rate decreases.

Also, without the motor driven compressor 37A, the pressure of theexcess air stream 36 directed to the heat exchanger 52 is lower comparedto the pressure of excess air stream 36 in FIG. 2 . This means that theliquid air condensing pressure will also be lower, probably subcritical.As a result, each expander 35, 40 in the embodiment of FIG. 3 must bedesigned to operate at a somewhat lower temperature than correspondingexpander in the embodiment of FIG. 2 . Depending on the liquid make flowrequirements and the product nitrogen pressures, the waste and excessair expander 40 in any of the disclosed configurations may operate at asufficiently low temperatures that the waste exhaust stream 49 may bereconfigured to feed the cold end of the subcooler 53.

The embodiment depicted in FIG. 4 shows a further arrangement of thenitrogen producing air separation unit 12 where both turbine-boosterarrangements (37D, 40) and (37C, 35) operate at near ideal designparameters so that they can achieve good efficiency with no gearing.Again, as many of the features, components and streams associated withthe nitrogen producing air separation unit 12 shown in FIG. 4 aresimilar or identical to those described above with reference to theembodiment of FIGS. 2 and 3 , the associated descriptions will not berepeated here. Similarly to the embodiments of FIGS. 2 and 3 , thecolumn pressure in the arrangement depicted in FIG. 5 has been set todeliver the gaseous nitrogen product 57 to the customer without furthercompression (i.e. without the need for a nitrogen product compressor).

The first key difference is the booster compressor 37D powered by thewaste and excess air expander 40 further compresses the entirecompressed, purified air stream 29, rather than just the excess airstream. Now booster compressor 37B is designed or configured to handlemuch more flow and creates a lower pressure ratio. As a result, theoptimal speed of booster compressor 37B is much lower than the speed ofthe booster compressors coupled to the waste and excess air expanderdepicted in FIGS. 2 and 3. More importantly, the speed of boostercompressor 37B is comparable or generally matches the optimal speed ofwaste and excess air expander 40. The concomitant benefits ofeliminating the gearing are less mechanical losses and some capital costreduction.

With the lower pressure excess air stream 31 exiting the first boostercompressor 37D, the motor driven compressor 37A is needed to raise thepressure of the excess air stream unless the design liquid rate issignificantly lower than about 60% of the total nitrogen product, and/orthe product pressure is higher than about 120 psig. For a liquid ratemake of about 60%, the motor driven compressor 37A depicted in FIG. 5will preferably be one or two stages.

The operational liquid turndown range of the arrangement depicted inFIG. 4 will be more limited than that of the other configurations,especially when no product gas nitrogen compression is used. This isbecause the discharge pressure of the main air compressor 24 mustincrease as the liquid production rate is decreased in order to providethe air stream 33 from the first booster to the distillation column 65at sufficient pressure. To turn down the liquid production rate orliquid make rate, the excess air stream 31 flow is decreased. This willreduce the flow of the compressed, air stream 26 from main aircompressor 24. But the lower excess air stream flow will also tend toreduce the discharge pressure of the motor driven compressor 37A. Also,with the lower excess air stream flow, the power required to drive thebooster compressors is less so the booster compressors will generateless pressure rise even though they are compressing less flow. This isunlike the other illustrated embodiments, where the discharge pressurefrom main air compressor 24 will decrease slightly as the liquidproduction rate is decreased, which benefits turndown operation. The useof a variable speed drive such as a direct drive motor for the motordriven compressor can resolve this issue for this configuration if awide liquid turndown range is important.

FIG. 5 depicts yet another embodiment of the present system and methodthat incorporates a nitrogen-based refrigeration cycle that includes anitrogen recycle compressor and a recycle nitrogen expander. Again, asmany of the features, components and streams associated with thenitrogen producing air separation unit 15 shown in FIG. 5 are similar oridentical to the embodiment of FIGS. 2-4 , the associated descriptionswill not be repeated here. Also, similarly to the embodiments of FIGS.2-4 , the gaseous nitrogen product 57 is delivered to the customerwithout the need for a nitrogen product compressor.

In the embodiment of FIG. 5 , the excess air stream 31 is a divertedportion of the compressed, purified air stream 29 and is combined with awarmed waste stream 46. The warmed waste stream 46 is raised in pressureto the same pressure as the excess air stream 31 by boosting it inbooster compressor 37B after it has been fully warmed in the main heatexchanger 52 using the work energy from the waste and excess airexpander 40. Since this booster compressor 37B must only raise thepressure of the warmed waste stream 46 to match the pressure of theexcess air stream 31, only a fixed amount of excess air is passedthrough the waste and excess air expander 40 to power the boostercompressor 37B. A gear may be needed between the waste and excess airexpander 40 and the booster compressor 37B to enable good aerodynamicefficiencies of the machines.

The combined excess air and waste stream 48 is then directed to thewaste and excess air expander 40 where it is expanded to form a wasteexhaust stream 49 that is warmed in heat exchanger 52. In this manner, aportion of the refrigeration for the air separation unit 10 is thusprovided by the expansion of the combined excess air and waste stream 48in expander 40. The warmed waste stream 41 exits the warm end of theheat exchanger 52 and may be used as a purge gas stream duringregeneration of the adsorbents and other layers in the pre-purificationunit 28.

Because the excess air stream 31 is only provided at a pressure asimilar to the discharge pressure of the main air compressor 24 andexpanded once across a single turbine/expander, the supplementalrefrigeration provided by the excess air circuit is limited. Theremainder of the supplemental refrigeration required for high liquidnitrogen production is provided by a nitrogen-based refrigerationcircuit. The nitrogen-based refrigeration circuit includes a nitrogenrecycle compressor 97A, a booster compressor 97C and a recycle nitrogenexpander 95. A portion of product nitrogen 57 may be provided to thenitrogen-based refrigeration circuit together with a nitrogen recycleexhaust stream 98 and is compressed in recycle compressor 97A andfurther compressed in booster compressor 97C, with aftercoolers 99A and99C used to remove some of the heat of compression. The compressed,cooled nitrogen recycle stream 96 is then directed to the heat exchanger52 where the stream is split. A first portion of the compressed, coolednitrogen recycle stream 91 is expanded in the nitrogen recycle expander95 with the resulting nitrogen recycle exhaust stream 98 being warmed inheat exchanger 52 and recycled to the nitrogen recycle compressor 97A. Asecond portion of the compressed, cooled nitrogen recycle stream 92 isfully cooled in the heat exchanger 52 to produce a liquid nitrogen feedstream 92 that is directed to the distillation column 65. In thismanner, the nitrogen recovery is enhanced, in part, because the streamof liquid nitrogen 92 is supplied to a location proximate the top of thedistillation column 65 in the illustrated single column arrangementrather than the liquid air stream 55 in the previously disclosedembodiments. In a dual column arrangement the stream of liquid nitrogen92 may be supplied to a location proximate the top of the higherpressure column.

Advantages of the air separation unit arrangement depicted in FIG. 5include the capability of a wider and more efficient turndown operationwithout sacrificing nitrogen recovery. Also, the net power consumptionof the embodiment depicted in FIG. 5 is comparable to the powerconsumption of the prior art arrangement depicted in FIG. 1 . Nitrogenrecovery is enhanced, in part, because the stream of liquid nitrogen 92is supplied to the distillation column 65 rather than liquid air stream55 in the previously disclosed embodiments in FIGS. 2-4 .

Turning now to FIGS. 6 and 7 , there are shown schematic diagrams of yetfurther embodiments of the present system and method. In theseembodiments, the waste and excess air expander is generated loadedrather than boosted loaded. Such arrangements are suitable forapplications where gearing between the booster compressor and waste andexcess air expander is to be avoided.

Specifically, FIG. 6 is an alternative arrangement of the nitrogenproducing air separation unit 16 to that shown and described above withreference to FIG. 2 with a generator 140 loaded excess air and wasteexpander 40. Without the booster compressor 37B coupled to the waste andexcess air expander 40, the liquid making capability of the airseparation cycle may be reduced. However, to mitigate or amelioratedthis issue, the discharge pressure from the motor driven compressor 37Ais increased and/or the flow of the excess air stream 31 is increased. Ahigher discharge pressure from the motor driven compressor 37A mayrequire an additional compression stage. Alternatively, one can keep theexisting motor driven compressor 37A and the flow of the excess airstream 31 similar to the embodiment of FIG. 2 but increase the dischargepressure from the main air compressor 24.

Similarly, FIG. 7 illustrates an alternative arrangement of the nitrogenproducing air separation unit 17 to that shown and described above withreference a generator loaded waste expansion version of the FIG. 3 .Since this embodiment does not have a separate, motor driven compressor,this arrangement of the will be most applicable to lower liquid productrate applications. However, liquid production can be increased in thisconfiguration, again by increasing the flow of the excess air stream 31flow or by increasing the discharge pressure of the main air compressor24, if that is feasible or possible.

It should also be pointed out that for lower liquid rate applicationsmost suitable for the embodiments of FIGS. 6 and 7 , with relatively lowflows of the excess air stream 31, it may be preferable to operate theexcess air expander 35 at or near ambient temperatures. In other words,the boosted excess air stream 36 would be passed directly to the excessair expander 35 from the aftercooler 39C without any further cooling inthe heat exchanger 52. With lower excess air flow rates this can be donewithout the occurrence of an excessively large warm end temperaturedifference in the heat exchanger 52. Hence, the penalty for lostrefrigeration at the warm end of the heat exchanger 52 is not large. Inthis way, a further benefit for operating the excess air expander 35 atan even warmer temperature, with the subsequent increase inrefrigeration production is exploited.

As suggested above, the present system and method are equally applicableto single column nitrogen producing air separation units and dual columnnitrogen producing air separation units. For example, the presentnitrogen producing air separation unit may be configured with a singlenitrogen column and single nitrogen condenser as generally shown anddescribed with reference to FIGS. 2-7 . Alternatively, the presentnitrogen producing air separation unit may be configured to include twonitrogen columns and the at least one nitrogen condenser. Still further,as shown in FIG. 8 , the present nitrogen producing air separation unit18 may comprises two or more nitrogen columns, including a lowerpressure column 170 and a higher pressure column 165 linked in a heattransfer relationship with a main condenser-reboiler 275 together withadditional nitrogen condenser 175, and streams 167, 168, together withoptional pump 177 to supply a reverse reflux nitrogen stream 178 fromthe top nitrogen condenser 175 to the higher pressure column 165 so thatthe nitrogen comes out at a single pressure. This would be theconfiguration needed to avoid any product compression. The higherpressure column 165 of the two column distillation column arrangementtypically operates in the pressure range of between about 8.5 bar(a) toabout 17 bar(a).

While the present nitrogen producing air separation unit capable ofproducing high pressure gaseous nitrogen without the use of a nitrogenproduct compressor and also capable of producing high rates of liquidnitrogen without adding additional compression stages in the feed aircompressor has been described with reference to several preferredembodiments, it is understood that numerous additions, changes andomissions can be made without departing from the spirit and scope of thepresent inventions as set forth in the appended claims.

For example, a variation of the embodiments of FIG. 2 or FIG. 4 may usean integrally geared ‘bridge’ machine that operatively couples all theturbine/expander and compressor stages in the excess air circuit. Theintegrally geared ‘bridge’ machine preferably drives compressor 37A, andthe two booster compressors 37B and 37C from the collective work orenergy supplied by the motor, the excess air expander 35, and waste andexcess expander 40. The integrally geared ‘bridge’ machine typicallyincludes a large diameter bull gear with several meshing pinions uponthe ends of which the various compression impellers are mounted formingthe plurality of compression stages. The pinions may also have differingdiameters to best match the speed requirements of the coupledcompression impellers and expanders. Since the excess air expander 35and booster compressor 37C operate optimally at about the same speeds,they will preferably be driven from the same pinion.

Another contemplated variation would be the design of the various heatexchangers. The liquid nitrogen subcooler 80 as well as the subcooler 53and the heat exchanger 52 are separate heat exchangers as generallyshown and described with reference to FIGS. 2-7 . In practice, thesecomponents may be separate heat exchanger cores or may be integrated sothat the liquid nitrogen subcooler 80 is combined with the subcooler 53.Alternatively, subcooler 53 may be integrated with heat exchanger 52 orboth subcoolers 53 and 80 may be combined with the heat exchanger 52.

What is claimed is:
 1. A nitrogen producing air separation unitconfigured to receive an incoming feed air stream and produce a highpressure gaseous nitrogen product and a liquid nitrogen product, andfurther configured to be capable of taking more than 30% of the nitrogenproduct as liquid nitrogen product, wherein the nitrogen producing airseparation unit comprises: a main air compression and purificationsystem; a heat exchanger; a distillation column system having at leastone nitrogen column and at least one nitrogen condenser configured tooperate at a pressure of about 8 bar(a) or higher; and a nitrogenliquefaction system, the nitrogen producing air separation unitcharacterized by: an excess air stream diverted from the incoming feedair stream at a location downstream of the main air compression andpurification system; one or more excess air compressors configured tofurther compress the excess air stream to a pressure greater than about24 bar(a), wherein a first portion of the further compressed excess airstream is directed to a nitrogen column; an excess air expanderconfigured to expand a second portion of the further compressed excessair stream to produce an excess air exhaust stream; and a waste andexcess air expander configured to receive a vapor stream from thenitrogen condenser and the excess air exhaust stream and expand thevapor stream from the nitrogen condenser and the excess air exhauststream to produce a waste exhaust stream; wherein the waste exhauststream is configured to be warmed in the heat exchanger against one ormore portions of the compressed, purified air stream to produce a warmedexhaust stream.
 2. The nitrogen producing air separation unit of claim 1wherein the one or more excess air compressors further comprise a motordriven booster compressor configured to receive and further compress theexcess air stream and one or more booster compressors driven by thewaste and excess air expander and excess air expander.
 3. The nitrogenproducing air separation unit of claim 2 wherein one or more excess aircompressors configured to further compress the excess air stream to apressure between about 34 bar(a) and 55 bar(a).
 4. The nitrogenproducing air separation unit of claim 1 wherein the one or more excessair compressors further comprise a first booster compressor operativelycoupled to and driven by the waste and excess air expander and a secondbooster compressor arranged in series with the first booster compressor,the second booster compressor operatively coupled to and driven by theexcess air expander.
 5. The nitrogen producing air separation unit ofclaim 4 wherein one or more excess air booster compressors configured tofurther compress the excess air stream to a pressure between about 24bar(a) and 35 bar(a).
 6. The nitrogen producing air separation unit ofclaim 1 further comprising a generator operatively coupled to the wasteand excess air expander.
 7. The nitrogen producing air separation unitof claim 1 further comprising a feed air booster compressor downstreamof the main air compression and purification system and configured tofurther compress the incoming feed air, the feed air booster compressoroperatively coupled to and driven by the waste and excess air expander.8. The nitrogen producing air separation unit of claim 7 wherein the oneor more excess air booster compressors further comprise a first boostercompressor configured to receive and further compress the excess airstream and a second booster compressor arranged in series with the firstbooster compressor, the second booster compressor operatively coupled toand driven by the excess air expander.
 9. The nitrogen producing airseparation unit of claim 1 further comprising a bridge machineoperatively coupled to the one or more excess air booster compressors,the waste and excess air expander and the excess air expander.
 10. Thenitrogen producing air separation unit of claim 1 wherein the at leastone nitrogen column comprises one nitrogen column and the at least onenitrogen condenser comprises one nitrogen condenser.
 11. The nitrogenproducing air separation unit of claim 1 wherein the at least onenitrogen column comprises two nitrogen columns and the at least onenitrogen condenser comprises one nitrogen condensers.
 12. The nitrogenproducing air separation unit of claim 1 wherein the at least onenitrogen column comprises two nitrogen columns and the at least onenitrogen condenser comprises two nitrogen condensers.
 13. A nitrogenproducing air separation unit configured to receive an incoming feed airstream and produce a high pressure gaseous nitrogen product and a liquidnitrogen product, and further configured to be capable of taking morethan about 30% of the nitrogen product as liquid nitrogen product,wherein the nitrogen producing air separation unit comprises: a main aircompression and purification system; a heat exchanger; a distillationcolumn system having a nitrogen column and a nitrogen condenserconfigured to operate at a pressure of 8 bar(a) or higher; and anitrogen liquefaction system, the nitrogen producing air separation unitcharacterized by: an excess air stream diverted from the incoming feedair stream at a location downstream of the main air compression andpurification system; a booster compressor configured to receive a vaporstream from the nitrogen condenser that has been warmed and to furthercompress the warmed vapor stream; an excess air expander configured toreceive the warmed, further compressed vapor stream and the excess airstream and to expand the warmed, further compressed vapor stream fromthe nitrogen condenser and the excess air stream to produce a wasteexhaust stream; and a nitrogen recycle circuit comprising one or morenitrogen recycle compressors and a nitrogen recycle turbo-expander, theone or more nitrogen recycle compressor stages configured to furthercompress a nitrogen recycle stream comprised of a diverted portion ofthe high pressure gaseous nitrogen product and a warmed nitrogen exhauststream to produce a further compressed nitrogen recycle stream andwherein a first portion of the further compressed nitrogen recyclestream is directed to the nitrogen column and a second portion of thefurther compressed nitrogen recycle stream is directed to the nitrogenrecycle turbo-expander configured to expand the second portion of thefurther compressed nitrogen recycle stream to produce a nitrogen exhauststream; wherein the waste exhaust stream is configured to be warmed inthe heat exchanger against the incoming feed air stream and the excessair stream to produce a warmed waste exhaust stream and the nitrogenexhaust stream from the nitrogen recycle turbo-expander is configured tobe warmed in the heat exchanger against the incoming feed air stream andthe excess air stream to produce the warmed nitrogen exhaust stream. 14.A method of providing supplemental refrigeration in a nitrogen producingair separation unit, wherein the nitrogen producing air separation unitis configured to receive an incoming feed air stream and produce a highpressure gaseous nitrogen product without a nitrogen product compressorand a liquid nitrogen product, the method comprises the steps of:compressing the incoming feed air stream in a main air compressor toproduce a compressed feed air stream; purifying the compressed feed airstream in a pre-purification unit to produce a compressed, purified airstream; splitting the compressed, purified air stream into an excess airstream and a compressed, purified feed air stream; cooling compressed,purified feed air stream in a heat exchanger to produce a fully cooledfeed air stream that is directed to a distillation column system;further compressing the excess air stream in one or more excess airbooster compressors to a pressure greater than about 24 bar(a); coolinga first portion of the further compressed excess air stream in a heatexchanger to produce a liquid air stream that is directed to adistillation column system; expanding a second portion of the furthercompressed excess air stream in an excess air expander to produce anexcess air exhaust stream; combining the excess air exhaust stream witha waste stream from the distillation column system to produce a combinedexcess air and waste stream; expanding the combined excess air and wastestream in a waste and excess air expander to produce waste exhauststream; and warming the waste exhaust stream in the heat exchanger toprovide supplemental refrigeration to cool the first portion of thefurther compressed excess air stream and cool the fully cooled feed airstream while producing a warmed waste exhaust stream.
 15. The method ofproviding supplemental refrigeration in a nitrogen producing airseparation unit of claim 14, wherein the distillation column systemfurther comprises at least one nitrogen column and at least one nitrogencondenser.
 16. The method of providing supplemental refrigeration in anitrogen producing air separation unit of claim 15, wherein thedistillation column system further comprises two nitrogen columns and atleast one nitrogen condenser.
 17. The method of providing supplementalrefrigeration in a nitrogen producing air separation unit of claim 15,wherein the distillation column system further comprises a lowerpressure nitrogen column and a higher pressure nitrogen column and twonitrogen condensers.
 18. The method of providing supplementalrefrigeration in a nitrogen producing air separation unit of claim 15,wherein the waste stream from the distillation column system is a vaporstream from a nitrogen condenser of the distillation column system. 19.The method of providing supplemental refrigeration in a nitrogenproducing air separation unit of claim 15, wherein the waste stream fromthe distillation column system is a warmed vapor stream from a nitrogencondenser of the distillation column system.